(a) Field of the Invention
The present invention relates to a dry developing powder. To be precise, it relates to a dry developing powder which is useful for electrophotography, electrostatic printing process, electrostatic recording process, magnetic printing process, etc. and facilitates pressure fixing and/or thermal fixing.
(B) Description of the Prior Art
As one of the fixing methods suitable for electrophotography, electrostatic printing process, electrostatic recording process, magnetic printing process, etc., there is known the so-called "pressure fixing method" in which a recording material carrying a developing powder in the shape of the image is introduced between a pair of pressure-applying rollers. This fixing method is said to be advantageous in that it can be instantly practiced by switching on the power source of the apparatus concerned, it is free of such troubles as environmental pollution, and it can perform the fixing practically completely on the occasion of continuous operation at high speed.
As to the dry developing powder (which hereinafter may be occasionally called "the developer" for short) for use in this pressure-fixing method, there has been proposed, for instance, a microcapsule toner in Japanese Patent Open Nos. 71648/1973 and 75032/1973. However, this developing powder is still unsatisfactory inasmuch as the coating resin therefor is restricted by both the tendency to electrification by friction with the carrier particles (i.e., triboelectric tendency) and the manufacturing technique.
There has also been proposed in Japanese Patent Open No. 50042/1975 a developing powder consisting essentially of resin, wax ingredient, pulverized magnetic substance and conductive particles. However, this developing powder, like the thermal-fixing developer described in the specification for U.S. Pat. No. 3,639,245, is one prepared by sticking conductive particles to the resin constituting the matrix of the powder by way of only the surface portion of said matrix. A developer of such a composition has drawbacks such that in the case where it is employed in forming an image, although the fixing property thereof is satisfactory, the fluidity of the developing powder is insufficient and the preservability thereof is poor. Besides, such a developing powder tends to be attended with phase separation of the wax ingredient and the thermoplastic resin at the time of manufacturing, which hampers easy manufacture thereof.
On the other hand, in the case of the thermal-fixing method wherein the fixing is effected by melting a portion of the binding agent by applying heat, there has not yet been developed such a developer that can be manufactured with ease and that displays an excellent continuous gradation of image.